Current radio communication systems conventionally use, for transmitting digital data coding an audio signal or, more generally, information of any nature, so-called constant-envelope modulations. With such modulations, the data transmitted are not carried by the amplitude of a radiofrequency carrier but by its phase or its frequency. This makes it possible to use in the transmitter a radiofrequency power amplifier operating in an operating zone close to saturation. Thus, the transmitter exhibits high efficiency in terms of power, this being required especially within the framework of using the transmitter in portable radio communication equipment. Specifically, as is known, in such an operating zone the transmitter exhibits amplification nonlinearities comprising amplitude nonlinearities and, to a lesser extent, phase nonlinearities. These nonlinearities create amplitude and phase distortion of the signal transmitted, which degrade the performance of the transmitter in terms of transmission quality. It is in order to circumvent the problem of amplitude nonlinearities that only constant-envelope modulations are used in current radio communication systems. Specifically, since the useful information is not carried by the amplitude of the radiofrequency signal transmitted, the amplitude nonlinearities do not affect the quality of transmission.
However, it is currently sought to transmit more information inside a frequency band of given width, assigned to a transmission channel, so as to increase the spectral efficiency of radio communication systems. The aim is to respond to the rise in traffic demand within the radiofrequency spectrum while complying with the constraints related to the sharing of this spectrum. This is why the reintroduction of an amplitude modulation is envisaged. Thus, it is sought to perfect new radio communication systems using a composite modulation, comprising both a phase modulation component and an amplitude modulation component, for the transmission of information.
Despite this, the need to maintain high transmitter efficiency in terms of power, is reason to continue to operate the radiofrequency power amplifier in an operating zone close to saturation. It is therefore desirable to nullify the effects of the amplification nonlinearities induced by the radiofrequency power amplifier, so as not to degrade the quality of transmission.
Several techniques are known for nullifying the effects of amplification nonlinearities. In the CLLT technique (standing for “Cartesian Loop Linear Transmitter”), a demodulator is used to servo control, in baseband and in analog, the modulation of the signal present at the level of the antenna with that of the baseband signal to be transmitted. In the ABP technique (standing for “Adaptive Baseband Predistortion”), a digital processing applied to the samples of the baseband signal to be transmitted makes it possible to distort this signal so as to obtain the desired radiofrequency signal at the level of the antenna. However, the implementation of these known techniques within the framework of a composite modulation is complex and necessitates thorough modification of the architecture of current transmitters.
This is why the present invention proposes a device for producing a phase and amplitude modulated radiofrequency signal relying on a technique known by the name of the EER technique (standing for “Envelope Elimination and Restoration”). According to this technique, the device comprises means of composite coding making it possible to generate on the basis of the data to be transmitted a first string of digital values corresponding to a phase modulation component of the output signal G and a second string of digital values corresponding to an amplitude modulation component of this signal. These two components are then transposed into the radiofrequency domain by distinct means. The components thus transposed are then combined to form the output signal G. Stated otherwise, the EER technique does not comply with the conventional scheme consisting in generating a phase and amplitude modulated baseband signal then in transposing this signal into the radiofrequency domain. This technique is advantageous since it makes it possible to retain a transmitter architecture close to that used within the framework of current systems using constant-envelope modulation.
The diagram of FIG. 1 gives an illustration of a device for generating a phase and amplitude modulated radiofrequency signal relying on the EER technique. The device comprises at least one data input 10 for receiving a digital message A containing data to be transmitted. It furthermore comprises composite means of coding such as a COD coder for generating, on the basis of said digital message A, a first string of digital values B and a second string of digital values C. The string B corresponds to a phase modulation component of the output signal G of the device. The string C corresponds to an amplitude modulation component of the output signal G. The device also comprises means for generating a phase control signal D and an amplitude control signal F on the basis of the first string of digital values B and of the second string of digital values C. The signals D and F are for example analog signals delivered by digital analog converters (not represented) of the generating means GEN. The digital values of the first string B and of the second string C are processed as samples of the phase control D and amplitude control F signals respectively.
The device also comprises phase modulation means MOD, an input of which receives the phase control signal D and an output of which delivers a radiofrequency signal of substantially constant amplitude and phase modulated E as a function of the phase control signal D. The device finally comprises a variable-gain radiofrequency power amplifier PA, the input of which is coupled to the output of the phase modulation means MOD so as to receive the radiofrequency signal E, a gain control input of which receives the amplitude control signal F, and an output of which delivers a phase and amplitude modulated radiofrequency signal. The signal G is the output signal from the device. This signal is suitable for radio transmission via an antenna. Thus, the output of the amplifier PA can be coupled to an antenna 30 for radio transmission of the signal G.
Since the phase modulation component and the amplitude modulation component are transposed from the baseband to the radiofrequency domain by distinct means, namely by the modulation means MOD and by the variable-gain radiofrequency power amplifier PA respectively, the phase modulation component and the amplitude modulation component follow different routes before being combined in the output signal G. Now, depending on the type of coding used, it is necessary for the phase modulation component to be synchronized with the amplitude modulation component in the signal transmitted, so as to allow correct decoding receiver-side and satisfactory spectral purity on transmission. It is therefore desirable to compensate for the differences in transmission times of these two components over their respective routes.